Gallium arsenide (GaAs), a III-V compound, is a material which has important specific optical and electronic applications. GaAs films are currently formed by chemical vapor deposition from the decomposition of an arsenic-containing compound and a gallium-containing compound. The arsenic-containing compound commonly used for forming gallium arsenic film is arsine, AsH.sub.3. AsH.sub.3 is highly toxic, the TLV in air of AsH.sub.3 being 0.05 ppm. Furthermore, it is a gas, the compound having a boiling point of -62.5.degree. C. Accordingly, it must be handled with extreme caution. The hazard presented by AsH.sub.3 is such that contemplated regulations proscribe its industrial use in the U.S. Finding a less hazardous substance than arsine, which may be used to deposit arsenic-containing films, is highly desirable, and may be absolutely necessary should the use of arsine be banned.
Potential alternatives for arsine itself are organoarsines. Although mono- and dialkylarsines are toxic, they are less so than arsine. Importantly, they are generally liquid or solid at room temperature, making them far safer to handle than arsine. Both diethylarsine, Et.sub.2 AsH, and mono tertiary butylarsine, t-BuAsH.sub.2, have been used as alternate arsenic sources with some degree of success. Diethylarsine is a liquid at room temperature and has a boiling point of 105.degree. C. Mono t-butylarsine is also a liquid with a boiling point of 65.degree.-67.degree. C. Other alkylarsines have sufficiently low boiling or sublimation points that they may be vaporized at combinations of temperature and pressure consistent with chemical vapor deposition.
A hindrance to the use of mono- and dialkylarsine for CVD or other deposition processes is the difficulty in purifying such compounds, particularly with respect to levels of silicon-, zinc- and germanium-containing compounds. Using currently available synthetic techniques and purification methods, mono- and dialkylarsines can generally achieve at best a level of silicon as low as about 200 to 300 ppm. For producing GaAs films useful for electronic purposes, an arsenic-containing compound having a silicon level of less than 5 ppm, and preferably less than 1 ppm, must be used. Trace levels of silicon are known to seriously impair the optical and electrical qualities of GaAs films. Applicants have found, for example, that fractional distillation of diethylarsine in a three-foot stainless steel packed column is inadequate to rid the compound of silicon impurities.
It is a general object of the present invention to provide methods of highly purifying mono- and dialkylarsines, particularly to substantially remove all trace silicon-containing, zinc-containing and germanium-containing impurities from the primary and secondary arsines.